The invention relates to a method for manufacture of a suitable substrate for the subsequent growth of a single crystal diamond layer and for the manufacture of a single crystal diamond layer, to a substrate as well as to a carrier wafer and a diamond jewel.
Single crystal diamond layers are particularly desirable for applications in high temperature electronics. Diamond is a crystalline high-pressure phase of carbon which is meta-stable under normal conditions. The stable phase is graphite. In addition to naturally occurring diamonds, diamonds are also produced artificially by a high-pressure method. These diamonds are normally very small and are used for grinding purposes because of the hardness of the diamonds. For electronic applications of diamonds, it is principally thin, single crystal diamond layers that are of interest. High-temperature applications which are made possible by the high band gap of diamond of about 5 eV are at the center of interest. Diamond can in principle be deposited epitaxially onto single crystal diamond by means of chemical vapor deposition (CVD) in a corresponding hydrogen atmosphere. The presence of hydrogen serves in this connection for the preferential etching away of the stable equilibrium phase in the form of graphite, which is likewise deposited. For practical applications, the epitaxy of diamond layers on single crystal diamond crystals is not of great importance, because only very small single crystal diamond substrates are available and because large area substrates of other materials with similar lattice constants to diamond do not exist. In the case of microelectronics and optoelectronics there are, however, semiconductor wafers which are commercially available in part with a diameter of up to 30 cm. Since no large area single crystal diamond substrates are available, numerous efforts have been made to produce single crystal diamond layers on other easily available substrates. The best success hitherto has been achieved with (100) orientated silicon substrates on which strongly textured, likewise almost (100) orientated diamond layers can be deposited by means of suitable CVD methods using an electrical voltage at the silicon substrate. These diamond layers consist of individual single crystal diamond grains in the micron range, which are twisted and tilted relative to the silicon substrate, with the twisting and tilting angles lying in the order of magnitude of about 1xc2x0. So-called grain boundaries thereby arise at points at which individual diamond grains abut, and greatly impair the electronic characteristics of the diamond film. It would, in contrast, be desirable to avoid these grain boundaries in order to actually produce a single crystal diamond layer. These diamond layers are described in the article by X. Jiang et al., Appl. Phys. Lett. 62 (1993) 3438.
It is an object of the present invention to provide a method for the manufacture of a substrate for the growth of single crystal diamond layers and also corresponding substrates which make it possible to produce extended single crystal diamond layers by epitaxy, so far as possible without disturbing grain boundaries, and diamonds built up on this for electronic and/or other purposes, such as industrial cutting and grinding processes, or in the form of diamond jewels.
The method of the invention for the manufacture of a suitable substrate for the subsequent growth of a single crystal diamond layer is characterized by the following steps:
a) selection of a substrate of a mono-crystalline material having a fixed lattice constant (aSi) or with a layer consisting of such a material,
b) manufacturing either a strained silicon layer with foreign material atoms incorporated at substitutional lattice sites on the monocrystalline material of the substrate,
c) transferring the strained layer into an at least partly relaxed state in which it adopts by relaxation and through the selected foreign atom concentration a lattice constant (aSi(C) which satisfies the condition
n.aSi(C)=m.aD
xe2x80x83where n and m are integers, preferably different integers, and aD is the lattice constant of diamond, with the relaxed layer forming the substrate, for example the substrate surface, for the epitaxial growth of the diamond layer.
In particular it is proposed that carbon atoms should be used for the foreign material atoms and n should be selected=2 and m=3.
The method of the invention is based on the fundamental realization that the almost epitaxial alignment of the diamond layers which can be grown on (100) silicon wafers is to be associated with the fact that the lattice constant of diamond aD has an almost rational relationship to the lattice constant of the silicon aSi, so that one can write
2aSi≅aD
The condition
2aSi=3aDxe2x80x83xe2x80x83(1)
is, however, not precisely satisfied. Furthermore, it has been speculated, in accordance with the invention, that if the corresponding relationship were precisely satisfied, one could expect direct epitaxial growth without misorientation and without grain boundaries.
In reality, 2aSi is approximately 1xc2xd% larger than 3aD. This is now seen as the reason why an adaptation arises with a corresponding faulty orientation (twisting and tilting) in the 1-degree region.
The basic concept of the present invention is that if one could reduce the lattice constant of silicon substrate by about 1%, so that 2aSi3aD, i.e. so that the condition (1) is precisely satisfied, the growth of single crystal diamond layers without substantial structuring should be possible on such a silicon substrate.
Furthermore, the invention recognizes that the desired reduction of the lattice constant of silicon by the incorporation of foreign material atoms on substitutional lattice sites of the crystalline silicon can be achieved. It is, for example, known that carbon as a group-IV-element can be electrically neutrally incorporated in crystalline silicon at substitutional lattice sites. Since carbon atoms are considerably smaller than silicon atoms, the incorporation of carbon leads to a volume reduction of the silicon crystal. In simplified manner, one can say that the volume of a silicon crystal reduces for each substitutionally incorporated carbon atom by an atomic volume xcexa9Si of the silicon. This is explained in more detail in the article by U. Gxc3x6sele in MRS-Proc. Vol. 59 (1986), pages 419 to 431.
Thus, a corresponding reduction of the average lattice constant aSi(C) results in dependence on the concentration Cc of the incorporated carbon.
The relationship
aSiC(Cc)≅aSi(1xe2x88x92xcex1Cc)xe2x80x83xe2x80x83(2)
applies approximately, with xcex1 having the value of 6,9xc3x9710xe2x88x9224 cmxe2x88x923. From this it can be calculated that a carbon concentration of approximately 2xc3x971021 cmxe2x88x923 (corresponding to approximately 1,5%) would be necessary in order to largely accurately satisfy the relationship (1). Since the diamond deposition takes place at elevated temperatures in the range of 800xc2x0 C., the different thermal expansion of diamond and silicon should also be taken into account, so that the relationship (1) applies at the deposition temperature and not necessarily at room temperature. The taking into account of the different coefficients of thermal expansion, however, only leads to a small modification of the carbon concentration that is required.
The solubility of carbon in silicon in thermal equilibrium is known and is extremely small (maximum about 1017 cmxe2x88x923) compared to the carbon concentration of approximately 2xc3x971021 cmxe2x88x923 required for the desired reduction in size of the lattice. It has, however, been shown that it is possible by means of both CVD processes and also by means of molecular beam epitaxy to grow carbon at these high concentrations (corresponding to a lattice contraction of ca. 2.5% and more) into epitaxial silicon layers in a meta-stable form, as can be found in the literature. In this connection reference is made to the following documents:
C. Guedj et al., J. Appl. Phys. 84 (1998) 4631,
W. Faschinger et al., Appl. Phys. Lett. 67 (1995) 2630,
A. R. Powell et al., J. Vac. Sci. Technol. B11 (1993:) 1064.
At higher temperatures and with correspondingly longer tempering times, substitutionally dissolved carbon precipitates out in the form of silicon carbide precipitations. For the handling of epitaxial growth processes, the epitaxial silicon layers with a carbon content are grown on silicon crystals, and a problem arises in that the carbon-rich silicon layers adopt the lattice constant of silicon parallel to the plane of growth and not the lattice constant which is to be expected from the relationship (2).
The carbon-rich layer is subjected to a high tensile stress, it represents a strained layer. A reduction of the tensile stress and an associated relaxation of the lattice constant of the layer by mismatched dislocations does not occur in practice, because the corresponding dislocations have only a negligible mobility in the carbon-rich material. At higher temperatures silicon carbide precipitation preferentially takes place instead of relaxation via mismatched dislocations. In order to overcome this problem, the invention provides that the strained carbon-rich silicon layer must be transferred into an at least partly relaxed state in which it adopts the desired lattice constant aSiC by relaxation and as a result of the selected concentration of foreign material. This can be realized when the carbon-rich layer is no longer mechanically fixedly connected to the silicon substrate on which it was grown, and a simple relaxation is thereby possible.
Since large area free-standing thin layers (with thicknesses typically under one micron) are not easy to handle technologically, it is desirable to reattach the correspondingly relaxed layers (with the now desired small lattice constant) to an appropriate carrier substrate. In this respect silicon is again preferred because it has practically the same thermal coefficient of expansion as the highly carbon doped silicon layer.
It is evident from the explanations given above how one can produce an initially strained carbon-rich silicon layer for the manufacture of a suitable substrate for the subsequent grown of a single crystal diamond layer and can then place this strained layer in a relaxed state so that it adopts a lattice constant (aSi(C)) which satisfies the condition 2(aSi(C))=3aD and also that the relaxed layer forms the substrate or at least the effective surface of a substrate for the epitaxial growth of the diamond layer.
It is not absolutely essential to use carbon atoms for the manufacture of the strained layer, but rather other foreign materials can be considered. It is also not essential for the substrate to consist of monocrystalline silicon, but rather other monocrystalline materials can also be used, providing the desired strained layer with the incorporated foreign material atoms on substitutional lattice sites can be grown thereon.
One possibility of placing the strained layer in a state in which it can relax by relaxation and adopt the desired lattice constant (aSi(C)) lies in producing a separation of the strained layer from the substrate. This can be achieved by etching trenches in a predetermined pattern into the strained layer and by etching away a layer arranged beneath the strained layer through the trenches by subsequent etching process by means of a suitable etching liquid. In this way the trenches are preferably so formed that square or rectangular regions of the strained layer arise.
It is particularly advantageous if, after the separation of the strained layer from the substrate by the etching liquid, the substrate with the now relaxed layer, which is still weakly coupled in place via the etching liquid, is removed from the etching liquid and the etching liquid present between the previously strained and now relaxed layer on the substrate is removed, whereby the relaxed layer comes to lie directly adjacent the remaining substrate and bonds with the latter in the relaxed state, for example through the formation of covalent bonds, whereby the remaining substrate becomes the carrier substrate.
Specifically, one can proceed in such a manner that one uses an SOI substrate in which a monocrystalline silicon layer is coupled via a silicon dioxide layer to an insulator, for example bonded to it, that the thickness of the silicon layer is reduced to a desired layer thickness dSOI, optionally by suitable thermal oxidation and then separation of the thermal oxide, for example by hydrofluoric acid, that a strained carbon-rich layer with the layer thickness dSi(C) is epitaxially grown onto the silicon layer, that the trenches are produced through the Si(C) layer and the lower lying Si layer by a dry etching process known per se which is highly selectively stopped at the silicon dioxide layer of the Si substrate, and that the separation of the strained layer from the insulator is carried out by dissolving the buried silicon dioxide layer of the SOI substrate in a bath with diluted hydrofluoric acid.
A silicon layer is preferably epitaxially grown onto the Si(C) layer prior to the etching of the trenches, and indeed with a thickness which corresponds essentially to the thickness of the Si layer lying beneath it, with both the thickness of the silicon layer beneath it and also the thickness of the cladding layer of silicon being selected to be substantially smaller than the thickness of the Si(C) layer, for example at least 10 times smaller and preferably orders of magnitude smaller. Since the Si(C) layer is arranged between two Si layers of at least substantially the same thickness, the strain which originates from these Si layers is relatively small, because the layers are thin. Furthermore, the presence of two Si layers of the same thickness on both sides of the Si(C) layer ensures that no bending deflection of the layer system takes place, which would be deleterious to the mechanical stability and the subsequent bonding process.
The dissolving of the silicon dioxide layer should in particular take place with a horizontal arrangement of the structure in the bath of dilute hydrofluoric acid because in this way the thin separated layer is supported carefully over its full area and any tendency of the separated layer to slide away from the lower lying substrate under the action of gravity is avoided.
It is not absolutely essential to use an SOI substrate for the epitaxial growth of the carbon-rich strained Si(C) layer, but rather one can grow this layer directly onto a monocrystalline silicon wafer as substrate.
The invention furthermore provides a method for the manufacture of a single crystal diamond layer which is characterized in that one grows the latter onto a relaxed layer which has been manufactured in accordance with one of the above-explained processes and which satisfies the corresponding condition naSi=maD.
After the manufacture of an epitaxial diamond layer without pronounced grain boundaries, the latter can be duplicated in accordance with a further development of the invention.
In other words, after the growth of a correspondingly thick single crystal diamond layer on a relaxed carbon-doped silicon substrate a thin film of this diamond layer is separated from the remainder of the diamond layer by a suitable hydrogen implantation process and bonded by a wafer bonding process to a carrier wafer. This process can be repeated multiply so that a plurality of carrier wafers with bonded on diamond layers can be produced from the original diamond layer. A further diamond layer can be grown epitaxially on each carrier wafer with a bonded on diamond layer and the hydrogen implantation process and wafer bonding process can be repeatedly used in order to produce even more carrier wafers with bonded on diamond layers. The original substrate with the relaxed carbon-rich silicon layer can also be multiply used or reused for the growth of further diamond layers which can then likewise be transferred by means of the hydrogen implantation process and the wafer bonding process to carrier wafers.
The method can also be carried out so that an epitaxial single crystal diamond layer in the layer thickness range above approximately 1 mm is produced over areas which are larger than approximately 1 square millimeter in order to realize the manufacture of diamond jewels.
The separation of the diamond layer in the region of the hydrogen-rich buried layer is preferably carried out in that the bonded wafer is held at a temperature of about 800xc2x0 C. for a sufficient time in order to produce microcrack formation in the hydrogen-rich layer (38) and subsequent splitting off of the carrier wafer (42) with the bonded on diamond layer (32A) along the microcracks and parallel to the substrate surface. The epitaxial production of the diamond layer is preferably carried out by a CVD process known per se.
The present invention has general validity in the sense that the method can be used for the manufacture of a suitable substrate for the subsequent growth of a single crystal material layer with any desired lattice constant aD. The general process is characterized by the following steps:
a) selection of a substrate of a monocrystalline material with a fixed lattice constant (aSi) different from aD or with a layer consisting of such a material,
b) production of a strained epitaxial layer of another material with foreign material atoms incorporated at substitutional lattice sites on the monocrystalline material of the substrate,
c) transfer of the strained layer into an at least partly relaxed state in which it adopts through relaxation and through the selected foreign atom concentration a lattice constant (aSi(C)) which satisfies the condition
n.aSi(C)=m.aD
xe2x80x83with n and m being integers, preferably different integers, and wherein the relaxed layer forms the substrate or substrate surface for the epitaxial growth of the material layer with the desired lattice constant aD.
For the sake of completeness reference should be made at this point to the article by Jean-Francois Damlencourt, Jean-Louis Leclercq, Michel Gendry, Michel Garrigues, Nabil Aberkane and Guy Hollinger with the title xe2x80x9cParamorphic Growth: A New Approach in Mismatched Heteroepitaxy to Prepare Fully Relaxed Materialsxe2x80x9d, in Japanese J. Appl. Pys., Volume 38, (1999) pages 996 to 999.
The method described there is concerned with the preparation of a thin relaxed starter layer (xe2x80x9cseed membranexe2x80x9d) (specific case described there In0.65Ga0.35As) with a fixed lattice constant on a monocrystalline substrate (in the specific case of InP) with a different lattice constant in order to then grow thicker layers of the same material (In0.65Ga0.35As) without strain on the thin relaxed starter layer.
For this purpose the starter layer is first epitaxially grown in the form of a thin pseudomorphic strained layer of a composition corresponding to that of the desired layer onto a sacrificial layer present on the substrate. The starter layer is subsequently separated from the substrate by removal of the sacrificial layer by chemical etching and is bonded in the relaxed state to the substrate again after the relaxation has taken place.
In the specific example the sacrificial layer is grown onto a buffer layer of AlInAs present on the substrate. For the chemical etching of the sacrificial layer trenches were etched through the starter layer and the sacrificial layer into the buffer layer in order to define square areas of the starter layer of 40xc3x9740 xcexcm2 to 300xc3x97300 xcexcm2 which were connected directly to the substrate via trampoline arms. The trampoline arms are on the one hand intended to enable the relaxation of the square regions of the starter layer, but on the other hand to ensure the retention of the crystallographic alignment of these regions with the substrate.
The process described there has also been further developed as can be read in the non-prior published article xe2x80x9cHigh-quality fully relaxed In0.65Ga0.35As layers grown on InP using the paramorphic approachxe2x80x9d by J. F. Damlencourt, J. L. Leclercq, M. Gendry, P. Regrany and G. Hollinger in Appl. Phys. Letter, 75, volume 23, of Dec. 6, 1999, pages 3638 to 3640.
Furthermore, the present invention includes a substrate which has been manufactured in accordance with one of the above-described methods, in particular a substrate which is suitable for the subsequent growth of a single crystal diamond layer on at least one of its surfaces with the special characteristic that a layer is present at the said surface of the substrate which has a lattice constant (aSi(C)) through relaxation of the selected foreign material concentration, which satisfies the condition n.aSi(C), with n and m being integers and aD being the lattice constant of diamond, with the relaxed layer forming the substrate for the epitaxial growth.
Furthermore, the present invention includes a carrier wafer with an epitaxial single crystal diamond layer bonded on via a bond layer and diamond jewels which can be produced from the diamond layers in accordance with the invention.
The invention will be explained in the following in more detail with reference to embodiments and to the drawings.